I'm slowly coming around from a period of ill health, so apologies for not posting before.
I think this is your best production yet. There is so much to like, contrasting sections, some strummed, some picked, cross melodies in different parts of the stereo field when listened to on headphones, a very good vocal, clear and not buried in the mix, but..............
There's something important missing. We know nothing about the father. We don't know what he was like or why he left. You might not think this is important, but hear me out and then decide.
My first impression when hearing it was the singer's like Miss Haversham in Great Expectations, waiting in her own world for something thats never going to happen. My second impression was that the father ran off with the blonde from the top of the street and deserted his family. It was only when I came to my third impression that the song started opening up. The letters could be advice about life from a father a) dying of cancer or b) serving in Afghanistan neither of whom would ever come home. The singer is then passing on his father's advice to his children. To me this version generates far more sympathetic feelings to the singer than the first two impressions. I stayed around for a third impression because I like a lot of your work. The lyrics don't have to be really explicit about what happened to the father, but they should give us a clue. I think the lyrics are good, just incomplete.
I suspect that the person you wanted to sing the song had similar views, the father didn't deserve the singer's feelings, making it difficult to sing.
If you have an i-pod or something similar, you could conduct an experiment. Put Yesterday and Something (both by the Beatles) into a play list. In the middle put the album version of Mother (John Lennon) and listen to them. Why do you think the Beatles songs are the most recorded songs of all time? Why do you think Mother isn't? Could you imagine George Harrison singing Mother? What about Frank Sinatra, or even Paul? Wouldn't it be strange if Yesterday was about Paul's mother, dressed up in such a way to disguise it? When writing songs for other people they need a reason to sing it. Yesterday shows you can sing about pain because being deserted by a girlfriend (boyfriend) is fairly universal. Not everybody has their mother run over by a policeman or is deserted by their father until he becomes rich and famous. However most Beatles fans would know what Mother is about because John told his story to millions before he even wrote the song, which makes listening to his emotions easier.
Finally you could do worse then listen (again?) to A Boy Named Sue by Johnny Cash. In my first and second impressions a similar song to yours, but it goes on to explain who the father is.
Hope you take my views as they are intended. I'll post some comments on mixing over the weekend.